A. Field of the Invention
The present invention relates to a MOS semiconnector device, more particularly a MOS semiconductor device having a MOS structure on one of its major surfaces, to control current flowing between electrodes on both of the major surfaces.
B. Description of the Prior Art
In recent years, there has been a strong market thrust for power switching elements with improved performances such as higher operating speed, breakdown voltage, and power handling capability. In light of this, because of good switching characteristics, MOS devices have been considered for such power switching elements; and considerable progress has been made in the MOS semiconductor technology.
In a vertical MOSFET, which has been a main technology for such power switching elements, the current, flowing in contact with both major surfaces of its semiconductor substrate, is controlled by a plurality of MOS structures on one of the major surfaces. The vertical MOSFET of this type is referred to as an insulated gate power MOSFET. In a conductivity modulation MOSFET, referred to as an insulated gate bipolar transistor, several layers, each of a different conductivity type, are formed on a major surface of the substrate opposite the surface having a MOS structure thereon, and the ON resistance of the MOSFET is reduced by conductivity modulation. Use of such a conductivity modulation MOSFET for power switching elements has gradually been increasing, particularly for switching power elements of the type requiring a higher breakdown voltage and larger current.
However, the above MOSFET devices have a problem in that a gate insulating film, which is formed between the gate electrode on the major surface, the MOS structure, and the source region contacting one end of the channel formed under the gate electrode, is easily destroyed by a surge voltage. The gate insulating film usually has a thickness of 500 to 1000 .ANG. and is easily destroyed by a surge voltage of 40 to 80 V. For this reason, much care had to be taken so as not to generate static electricity when handling such devices.
Some methods for protecting the gate insulating film against such surge voltages have been proposed in Japanese Patent Unexamined Publications Nos.: Sho. 58-87873, Sho. 58-178566, and Sho. 61-296770, for example. In these proposed methods, a bidirectional Zener diode is constructed in the polycrystalline silicon layer formed in the surface of the semiconductor substrate, and connected between the gate and source of the MOS device.
FIGS. 1 and 2 illustrate the structure of such a vertical MOSFET, as described in Japanese Patent Unexamined Publication No. Sho. 58-87873. Referring to FIG. 1, a p.sup.+ well 2 is formed in the surface region of an n-type silicon layer 1. An n.sup.+ source region 3 is formed in the surface region of p.sup.+ well 2. A gate 5 made of a polycrystalline silicon layer is formed over a gate oxide film 4, to form a channel between n-type layer 1, p.sup.+ well 2, and n.sup.+ region 3. A bidirectional Zener diode, which extends from gate 5, consists of a p region 71, and n.sup.+ regions 72 and 73 on each side of p-region 71, is formed on an insulating film 6 extending to gate oxide film 4.
The n.sup.+ region 72 and n.sup.+ source region 3 are connected to a source terminal S. The n.sup.+ region 73 and gate 5 are connected to a gate terminal G. An n.sup.+ layer 8, on the side of n-type layer 1 opposite the bidirectional Zener diode, is connected to a drain terminal D. A p layer 21 having an edge breakdown voltage structure is formed under the bidirectional Zener diode.
FIG. 2 is a perspective view of the bidirectional Zener diode portion of the vertical MOSFET of FIG. 1. FIG. 3 is a circuit schematic of the vertical MOSFET of FIGS. 1 and 2 which comprises a vertical MOSFET 31 and a bidirectional Zener diode 32 connected between the gate and source of MOSFET 31.
With a trend toward increasing operating frequencies and power handling capacity, surge voltages are also increasing. For example, in large power switching elements for automotive applications, the MOS device has to operate under most severe conditions; for example, the device must withstand surge voltages as shown in FIG. 4. Under these circumstances, the conventional surge protecting method of using the bidirectional Zener diode as surge protecting element (such as described above) would not provide sufficient protection from such large surge voltages. As a solution, increasing the effective area of the protecting Zener diode has been proposed. However, this approach increases manufacturing cost, because the area of the protecting Zener diode necessary to absorb such large surge voltages is considerably larger than a commercially desirable size.